Generally, since the resolution of a motion picture is higher than that of a still picture, a signal embedded in a motion picture must be weaker than is a signal embedded in a still picture so that image quality can be maintained. Even if a strong signal is embedded, it is considerably attenuated after being subjected to a low-pass filter or to MPEG2 compression. Therefore, in a method for detecting signals independently in each frame, or a method for detecting signals in a fixed number of frames, since the strength of the signals to be measured depends on the compression process, the reliability of the detection is not always high. If a threshold value for enhancing the reliability is set, another problem arises that embedded signals can not be detected at all (false negatives).
A method is required whereby the reliability of the detection does not depend on the strength of a signal that is measured in a single frame. However, means for resolving the problem are not taught in the “data hiding method and data extraction method employing statistical inspection” disclosed in Japanese Patent Application No. Hei 8-348426, the “data hiding method and system employing statistical property” disclosed in Japanese Patent Application No. Hei 8-345568, and the “multiple frame data hiding method and detection method” disclosed in Japanese Patent Application No. Hei 9-88493.